1. Field of the Invention
The present invention relates generally to a communications system between one or more remote terminals and a host terminal over a telecommunications transmission line for transmitting a plurality of channels of telecommunications information and which may also include a fiber optic transmission line. A plurality of control signals or sensed data are added and communicated in either direction on the transmission line utilizing a serial channel. The present invention contemplates an optical drop in the case of a fiber optic communications channel in which a control word may be added and multiplexed together with other channels for transmission over a fiber optic transmission line.
2. Description of the Prior Art
It is well known in telecommunications transmission and in particular in telecommunications transmission over fiber optic transmission lines to combine a plurality of channels such as, for example 672 voice (DS0) telephone channels including up to 28 DS1, 14DS1C or 7DS2 channels together in a digital multiplexer to multiplex and demultiplex the various combinations of DS1, DS1C and DS2 signals to and from a single DS3 signal for fiber optic transmission at a DS3 rate of 44.736 Mb/sec. Even higher fiber optic transmission rates of 143.36 Mb/sec. may also be utilized to multiplex and demultiplex up to 2,016 digitized voice channels or equivalent DS0 signals to and from the 143.36 megabit data stream. Between the communications terminals at either end of the fiber optic span, a drop and insert repeater may provide for the extension of fiber spans and access to other DS1 or equivalent DS1C or DS2 signals. A problem in the known prior art is that it is sometimes necessary, particularly in an add drop, configuration to add additional control information into the communications channels already multiplexed and transmitted over the telecommunications line while at the same time insuring that synchronization is maintained between the additional control information to be added and the extraction of the added control at the far end receiver such that synchronization is always maintained between the control word added and the transmitted channelized data regardless of whether or not the added control bits of the added control word are varied or changed over time and in particular over a very short time interval.